The present invention relates generally to electronic ballasts for powering lamps. More particularly, the present invention relates to program start lamp ballasts with protection circuitry and associated methods for protecting the ballasts from input arcing and line interruption.
Lamp ballasts are typically required to be fully functional, or to otherwise automatically restore lamps to an original operating state, whenever an input power line is arcing or has been interrupted. This is an important reliability feature for electronic ballasts generally, and is even more particularly so for program start ballasts. Good program start ballasts should always reliably start the lamp in accordance with an appropriate preheating process each time after powering off and on.
Referring to FIG. 1, a conventional program start ballast 10 may be described which is designed using an integrated-circuit (IC) driven voltage-fed, half-bridge inverter topology. An input voltage source Vac is coupled to four diodes D1-D4 in a bridge configuration defining an input AC-DC general rectifier and providing a DC output Vdc. A power factor correction (PFC) section is coupled to the diode bridge output and provides a constant DC voltage output V_rail through an electrolytic capacitor C1.
An inverter section includes a pair of switching elements Q1, Q2, which in the conventional example shown are MOSFETs in a half bridge configuration and a main series resonant tank to which one or more lamps La may be coupled to receive output power from the ballast. The resonant tank as shown is formed by a DC blocking capacitor Cdc, a resonant inductor Lrp, and a resonant capacitor Cres. Secondary windings of the resonant inductor Lrs1, Lrs2 are respectively coupled across filaments on either end of each lamp La.
A controller 14 is provided to drive the switching elements Q1, Q2. Capacitors C10, C11 and diodes D11, D12 collectively form a power supply circuit for the controller 14.
In a typical program start application of the ballast 10 as described above, the voltage V_rail charges capacitor C2 to a startup threshold voltage associated with the controller 14. The controller 14 then sets a preheat frequency by which the switching elements Q1, Q2 are driven to preheat the lamp filaments Rf1, Rf2 for a period of time T_pre. After the preheat time T_pre, the controller 14 begins sweeping the drive frequency continuously to ignite the lamp La by using the signal across capacitor C9 that is obtained by voltage sensing circuit block 12. When controller 14 detects breakdown and ignition of the lamp, the controller 14 begins steady-state lamp current control. During steady-state operation, the controller 14 additionally monitors the voltage across the capacitor C9 for overvoltage and end-of-life (EOL) conditions.
Typically the voltage supply Vcc for the controller 14 has two critical threshold voltages, V_startup and V_reset. V_startup is typically larger than V_reset. The voltage supply Vcc for the controller 14 has to be greater than V_reset to initialize the control sequence (e.g., preheat, startup, steady-state operation, abnormal condition control). The controller supply voltage Vcc must further be greater than V_startup to start the oscillations and driving of the switching elements Q1, Q2.
To fully reset the ballast, the controller supply voltage Vcc needs to drop below V_reset and then increase to or above V_reset to fully reset the controller 14 so that the controller 14 can initiate the normal startup sequence after the subsequent power-on. If the supply voltage does not start increasing from V_reset, the controller 14 will not go through the preheat sequence and start the lamp directly. Without the preheat sequence, the lamp may likely be more difficult to ignite, which requires the ballast to provide a much larger output voltage V_c_res to ignite the lamp without preheating the filaments. This large output voltage may cause the voltage across capacitor C9 to be greater than the EOL shutdown threshold voltage, and further cause the permanent shutdown of the controller 14. In this case, the only way to restart the lamp is to recycle the input power to the ballast or remove the lamp.
If there is a short line voltage interruption or occurrence of input arcing, the voltage supply to the controller 14 may not have sufficient time to drop below the reset voltage V_reset. The reason for this is that it takes a relatively long time to discharge the capacitor C1, which is a large electrolytic that can hold a great deal of charge. If the line interruption concludes and power is restored before the supply voltage drops below the reset voltage V_reset, then the main voltage V_rail is going to once again charge up capacitor C2 through resistor R3. As a result the ballast could start the lamp directly without any preheating of the filaments and may possibly shut down because of the large starting voltage needed to break down the lamp.
This shutdown condition can have serious consequences. For example, lightning strikes may cause a circuit breaker to shut off and on very quickly. The circuit breaker shut off/on can generate a line interruption or input arcing, and all of the ballasts linked in. for example. a commercial lighting application would then shut down at the same time during a typical lightning strike.
Therefore, what is needed is to design a protection circuit for abnormal conditions such as lightning strikes in the above example, whereby such a collective shutdown may be prevented. It would be further desirable to design such a protection circuit in a manner so as to minimize its implementation costs.